In image forming devices such as an electrophotographic device, an electrostatic recording device, and an electrostatic printing device, an image formation method which forms a desired image by developing an electrostatic latent image to be formed on a photo conductor with a toner for developing electrostatic images is widely performed. The method is applied to a copier, a printer, a facsimile and their composite machines thereof.
For example, in the electrophotographic device using electrophotography, in general, the surf ace of a photoconductor consisting of a photoconductive substance is uniformly charged by various means, and then an electrostatic latent image is formed on the photoconductor. Subsequently, the electrostatic, latent image is developed using a toner, and the toner image is transferred onto a recording material such as a sheet of paper, and the material is heated so as to fix the image, whereby a copy is obtained.
As for the toner used in an image forming device, in order to improve functions such as the charge stability and fluidity of toner, external additives, such as inorganic particles and organic particles having a particle diameter lower than that of colored resin particles (toner particles) are generally attached and added onto the surface of toner particles (external addition) to use.
In the toner obtained using the conventional external additive, it is sometimes hard to obtain a charge rising property of toner at an early stage of printing. Particularly, the charge rising property of toner is easily influenced by the usage environment in a severe environment such as a low temperature and low humidity environment or a high temperature and high humidity environment. As a result, the image quality is easily deteriorated by fog at the early stage, and an adverse effect is given to early-stage printing performance.
In the continuous printing process of a large number of sheets, mechanical stress in a development device (the number of times of contact of toner particles with each other is increased by stirring) causes tendency of burying of an external additive to the toner particle surfaces and/or defects of being liberated (detached) from the toner particle surfaces. It becomes difficult to impart a stable charging ability (charge stability) to the toner particles over time. As a result, thin line reproducibility of printing is decreased and image quality deterioration such as fog is caused. Thus, an adverse effect is given to printing durability performance, which is problematic.
Thus, there is a need for development of a toner in which at an early stage of printing, charge rising property is excellent even in usage in a severe environment, even if the number of times of contact of toner particles with each other is increased by stirring in the development device in the continuous printing process of a large number of sheets, burying of the external additive to the toner particle surfaces and/or defects of being liberated are not caused, a state in which the external additive is appropriately attached is maintained over time, and a stable charging ability (charge stability) can be applied to the toner particles. In the case of having a stable charging ability, even if durable printing is carried out, the change of conveyance property of toner is little, which is advantageous.
Patent Document 1 discloses a positively-chargeable toner comprising toner particles and a dry-type silica fine powder and a wet-type silica fine powder as external additives in order to provide a toner whose image density is stable over a long period of time and which causes no fog even in the case of forming images at a low printing rate, wherein the dry-type silica fine powder has a positively charged polar group and a hydrophobic group, the wet-type silica fine powder has a fluorine-containing negatively charged polar group and is surface treated with a quaternary ammonium salt silane compound.
Patent Document 2 discloses a positively-chargeable toner for developing electrostatic images, comprising colored resin particles and an external additive in order to provide a toner which imparts a stable charging ability and fluidity to toner particles over time, which has thin line reproducibility even if continuous printing of a large number of sheets is performed, and which hardly causes image quality deterioration due to fog in a high temperature and high humidity environment, wherein the toner contains, as an external additive, an external additive A (i.e., fatty acid alkali metal salt particles or fatty acid alkaline earth metal salt particles having a number average primary particle diameter of 0.1 to 1 μm) in an amount of 0.01 to 0.5 part by weight with respect to 100 parts by weight of colored resin particles; and an external additive B (i.e., spherical silica fine particles having a number average primary particle diameter of 40 to 200 nm and a sphericity of 1 to 1.3) in an amount of 0.2 to 2 parts by weight with respect to 100 parts by weight of colored resin particles.
However, the toner disclosed in Patent Document 1 cannot strike a balance between conveyance amount stability and characteristics in a high temperature and high humidity environment, whereas the toner disclosed in Patent Document 2 has excellent characteristics in a high humidity environment but has insufficient conveyance amount stability.